It is known that aircrafts are equipped with sockets called "ground connection", enabling the connection to external direct current or alternating current power units, called external direct current or alternating current units, for the powering of aircrafts with direct or alternating current, according to the different characteristics of voltage and frequency used for the equipment onboard the aircrafts, when the latter are at a standstill at airfields, the external power units being able to be at fixed points or, most often, movable because they are installed on service vehicles.
In comparison to aircraft, the principal advantage of helicopters is to be able to move from one point to another without the external infrastructure obligatory requirement for a runway, an electrical supply terminal or external starting unit.
This independence requires that a helicopter be equipped with onboard means enabling it to start independently its power unit, and in particular its turbine engine assembly with one or more turbine(s).
The specific uses of helicopters, leading them to take off outside airfields, thus makes it necessary to permanently provide, onboard each helicopter, a starting device comprising at least one energy source enabling the independent starting of the helicopter turbine engine unit.
Currently, energy sources enabling an independent start up of the turbine(s) of a helicopter are based on one of two types.div.
compressed gas tanks, generally of air, connected to both a dedicated installation and to at least one pneumatic starter, PA1 at least one battery of electrochemical accumulators, connected to both a dedicated installation and to at least one direct current electrical starter, this battery also being able to contribute to the electrical powering of other equipment on the helicopter.
In the second aforementioned case, the starting device can include at least one generator-starter, which is a reversible electrical starter which, once start up has been executed, powers the electrical network on board the helicopter, or at least one standard electrical starter (non reversible).
The diagram of the principle of a state of the art starting circuit for helicopter turbines is shown in FIG. 1. The curves characteristic both of the direct current starter motor torque of the circuit of FIG. 1 and of the resisting torque due to the turbine driven by this starter according to the rotational speed are shown in the graph in FIG. 2.
Onboard the helicopter, the outline of which is indicated by 1 in FIG. 1, the starting device includes a three-phase alternator 2, usually mechanically driven by the helicopter main transmission gear box, itself driven by the turbine engine assembly of this helicopter after its starting up, this alternator 2 being able to be connected, by the closing of a line contact 3, to a set 4 of three busbars for the distribution of three phase alternating current, to which can be connected, in parallel, an external alternating current power unit 5, by the closing of the contactor 6 of the alternating current ground connection socket. The contactor 3 is closed in the alternator mode when all the necessary conditions are met and the contactor 6 is closed in the "power unit" mode when all the necessary conditions are also met to supply the distribution terminals 4 with three phase alternating current, which terminals 4 themselves supply in parallel for example an air conditioning equipment and weapon's systems (not shown) of the helicopter, as well as a rectifier transformer 7 which transforms the three phase alternating current into direct current (28V) for the corresponding applications. In particular, the rectifier transformer 7 has its positive terminal connected by the rectifier transformer contactor 8 to a busbar 9 for distribution of the direct current, whereas its "-" terminal is permanently connected to a reference earth 10. In parallel, the direct current busbar 9 is connected, by the contactor 11, to the ground connection socket 12 to an external direct current power unit, the "-" terminal of this socket 12 being also permanently connected to the reference earth 10. Also in parallel, the direct current busbar 9 can be connected by the closing of the battery contactor 13 to the "+" terminal of an onboard battery 14, which is charged through the busbar 9 during normal operation after starting the turbine engine assembly, and which enables the independent starting of this assembly (not shown). The direct current busbar 9 itself powers the electric motor starter 15 by the closure of a starting contactor 16, which is a power contactor.
Since the direct current turbine starter 15 is a starter by nature not easily torque adjustable, two resistances 17 and 18 are mounted in series between the starter 15 and the reference earth 10 in order to avoid "the unloaded racing" of the starter 15, a second starting contactor 19 being connected in parallel to the resistance 17. Although the direct current starter 15 is always mechanically loaded by the resisting torque due to the driven turbine(s), its peak current when starting is such that it is necessary to limit it with the resistances 17 and 18, and therefore to limit the motor torque delivered by the direct current starter 15. According to whether it is closed or open, the contactor 19 activates either the single resistance 18 or that of the two series resistances 17 and 18, respectively.
By referring to the dashed line curve 20 of FIG. 2, expressing the motor torque of the direct current starter 15 as according to its rotational speed, it appears that if the current was not limited by the resistances 17 and 18, the starting torque (at zero speed) would be at level A and applied almost instantaneously, from which risks of breaking the transmission shaft. (not shown) connecting the starter 15 to the turbine. The contactor 19 being open, the function of the series resistances 17 and 18 is to restore the motor starting torque to the level B, the resisting torque then being zero. From the point B, the operating characteristic advances along BD. At the point D, the motor torque Cm becomes too weak relative to the resisting torque Cr associated with point D' on the continuous line curve 21. In order to avoid too significant a slow down, the resistance 17 is then short circuited by closing the contactor 19, 4 so that the motor torque characteristic Cm again rises to point C, towards the initial characteristic AZ, and the starting continues along the section CZ of the curve 20. The intensity of the direct current is from 1000 to 1200 A at levels B and C, under a base voltage of about 28 V which then decreases, and at D the intensity is of the order of 800 A.
Such starting is therefore very abrupt, and not very torque controllable, and the starting circuit requires conductors of significant sections, therefore heavy, considering the high intensity (of the order of 1200 A) of the direct current carried.
In short, a starting device with a series type of direct current starter motor, requires inserted electrical resistances and power contactors enabling the commutating of these resistances, as well as large diameter conductor cables (67 mm.sup.2 copper for example) for the power circuit, these cables not being able to be replaced by cables of more reduced diameter in a metal or alloy of less density than copper, such as aluminium, when the conductor cables are installed on the helicopter in locations, such as the transmission support platform, where the operating temperatures are high.
Such a starting device has the disadvantage that the motor torque supplied by the direct current starter is violent at starting and not very controllable and adaptable to the resisting torque, since the only regulation of the motor torque is obtained by the single regulation of the possible current by the commutation of the resistance 17. This results in two major constraints, which are a current of high intensity in the electrical network, from which the use, as mentioned above, of large diameter conductors, therefore heavier, and a mechanical stress on the whole of the mechanical linkage system, between the starter output shaft and the turbine, and more exactly its accessory box by which means the starting is carried out.